Such support jacks are used in particular to prop up the trailers of a tractor-trailer rig. These support jacks are made from rectangular tubes, placed one inside the other and able to move relative to each other, as is known for example from EP 1 104 369 B1.
To reduce the weight and the manufacturing expenses, a lifting mechanism with shaft tube and support tube is proposed in EP 0 972 688 A2, being fastened to a vehicle chassis by a mounting mechanism. The mounting mechanism should be configured as a mounting plate, closing an open U-shaped profile element, so that a hollow shaft tube is produced. For this, the mounting plate is welded to the U-profile by longitudinal welds for its entire length. The major drawback of this known lifting mechanism is the expensive fabrication, since the U-shaped profile element has to be welded to the mounting plate with very precise fit. Furthermore, there is the risk of a cross section of different clearance when there are welds situated on the inside, and this may cause hindrance in the extending and retracting of the support tube.
Another prior art is EP 0 380 941 A2. In the case of this support device, an outer sleeve is extruded as a single piece with its fastening plate for mounting the support device onto a vehicle. Since the rough casting is in block form, this method is confined to light metals, especially aluminum. But such support devices have not been successful on the market, since they are costly because of the high-value material and they have proven to be prone to breakdown in operational use, due to the low material strength.